The requirements imposed nowadays on diagnostic X-ray equipment of medium or high power cannot be satisfied economically and with up-to-date technology by using conventional regulating and high-tension trransformers operating on the mains frequency. In view of the high series reactances of low-frequency transformers, the task of establishing ever shorter exposure times, required by new diagnostic methods which involve the need of increasing the peak power, raises serious difficulties which beyond certain limits cannot be solved at all. This task necessitates both the temporary storage of electrical energy, which can be accomplished most economically with DC voltage, and an increase in the operating frequency of the inverter unit following the storage unit to obtain the required steepness of the current slope. With the aid of high-power semiconductor devices the practical realization of such a system is posssible. However, the radiographic and fluoroscopic modes of operation of diagnostic X-ray apparatus impose two distinct requirements. In one instance, i.e. in the case of radiography, the X-ray tube is loaded mostly for short periods of time with high power (on the order of 100 kW). In the other instance, i.e. during fluoroscopy, it usually operates continuously for a long time with low power corresponding nearly to the no-load condition. Therefore, to transform the stored energy to high voltage it is necessary to have a particular converter designed for such a dual operating mode instead of conventional semiconductor converters operating at a steady frequency and mostly with a given power output. This converter, on the one hand, should utilize the dynamic power range of the semiconductors while working at a low or medium frequency, higher than the mains frequency, over one or more operating periods measuring the required time of exposure; on the other hand, in the continuous fluoroscopy mode of operation near the no-load condition, it should work with minimum losses.
The output of the converter is loaded not only by the impedance present at the primary windings of the high-voltage or regulating transformers, corresponding to the adjusted operational parameters of the X-ray tube, but also by a transient reactance, resulting from the magnetic remanence of the iron core, which might multiply the current peak on start-up. The presence of this transient phenomenon necessitates a change in commutation energy because the power requirements of the two operational modes differ from each other by more than four decimal orders of magnitude.